The typical configuration of a gas line shut-off valve responsive to earthquakes rely essentially on a ball seated on a precarious perch just above a valve seat. The ball, when knocked off its perch by a tremor, falls into the valve seat thus interrupting the flow of gas. Various interpretations of this basic concept are disclosed in U.S. Pat. Nos. 2,585,316 Hobson, 4,331,171 Novi, 4,382,449 Nelson, 4,485,832 Plemmons et al., and 4,565,208 Ritchie et al. Extreme care must be taken in the installation of these valves to mount them in an absolute level position. Any tilting in any direction will directly affect the sensitivity of the valve. If the surface from which the ball is perched is not perfectly horizontal, the ball may have a tendency to be dislodged by minute shocks or vibrations, or, depending upon the construction of the perch, may fail to respond to seismic tremors. The orientation of the valve body may be directly affected by the position of the connecting pipes. Thus, this leveling requirement renders most valves of the prior art impractical for use on existing gas installations where the pipes have not been carefully leveled.
The sensitivity of a shut-off valve which relies on a perched ball is also affected by rust, scaling, debris accumulating around the base of the ball, and other long-term corrosive phenomena.